In conventional mammography, a projection image is created of the compressed breast in the craniocaudal or lateral direction. The image shows a “shadow image” of the breast. Generally, tungsten (W)—or molybdenum (Mo)—and rhodium (Rh)—anodes with high voltages in the range up to 35 kV, rarely up to 49 kV, are used. The low-energy component of the X-rays emitted, which mainly contributes to the X-ray dose applied, but only little to the imaging, can also be separated with Mo-, Rh- or Cu filters depending upon the application.
Beam guidance and scattered light filters ensure high spatial resolution. Old-fashioned X-ray film has mostly been replaced by digital detectors. U. Speck & I. von Brenndorff describe contrast media for use in projection mammography in the publication EP 0 994 729 B1. However, when conventional contrast media containing iodine (I) with the atomic number Z=53 as an opacifying element are used, the potential of the contrast medium can only be used to an unsatisfactory extent since the emitted X-ray spectrum only overlaps unsatisfactorily with the K-edge of the iodine at 33 keV. If higher order elements are used, for example gadolinium (Gd) with the atomic number Z=64 and a K-edge of 50 keV, the disadvantages of this low degree of overlapping of X-ray energy and Gd absorption peak are even more noticeable. For these reasons alone, the application of contrast media has not become established in the field of conventional projection mammography.
The above-described conventional projection mammography has experienced a series of improvements at all levels in the last decade. On the excitation side, monochromatic X-rays have been tested, wherein a reduction in the X-ray dose compared to the use of polychromatic radiation is possible with the same contrast-to-noise ratio (CNR). When X-ray tubes with two anodes are used, it is possible to use contrast media where the K-edge of the absorbing element lies exactly between the two emission lines of the two anodes so that switching between the anodes, “turns” the visibility of the contrast medium “on or off”. In principle, this enables the generation of subtraction images, which are virtually free of movement artifacts.
There have also been developments on the detector side—for example, nowadays energy-resolving detectors are available for a series of approaches. There has also been a gradual transfer to tomosynthetic representation by swiveling the tube and detector over the physiologically adapted clamped breast. At present, this development is tending toward CT devices that have been specially developed for breast imaging. As with conventional CT and MR imaging, in one possible embodiment, the woman lies on her stomach, only instead of the whole body, solely the breast is scanned without mechanical clamping. This produces 3D images such as those familiar from CT and MRI. However, specialization with regard to the breast enables a more economic and hence widely used device to be developed with high spatial and temporal resolution and used and marketed with a minimized X-ray dose. In this way, a device of this kind eliminates substantial drawbacks associated with conventional or traditional projection mammography, namely:                swinging from one breast to the other;        swinging from craniocaudal side to lateral;        mechanical compression of the breast;        movement artifacts, if the compression is changed or loosened in a sequence of two images;        no 3D images and        deficient temporal resolution for the use of contrast media in dynamic succession.        
In traditional projection mammography, the application of contrast media was a major exception since, due to the compression of the breast, intact inflow into tumors was not guaranteed. Relief of the compression after unenhanced imaging for the application of contrast media with subsequent recompression automatically resulted in enormous movement artifacts. It is precisely this problem that is avoided with the use of the described CT mammography systems since the mammographic images are now obtained with the woman in prone position with a non-compressed breast, which also enables contrast medium to be applied to the prone patient without problems. In addition, the embodiment of X-ray source, the beam guidance and X-ray detection with energy resolution result in previously unknown possibilities for contrast-medium adaptation and beam reduction.
However, there is still the significant problem that, with the previous imaging with contrast media, the X-ray energy regions of the X-rays used and the main absorption regions of the contrast media used were unsatisfactorily matched to each other.